How can I get bootstrap version via javascript?

You can use this code found here :

  var getBootstrapVersion = function () {
  var deferred = $.Deferred();

  var script = $('script[src*="bootstrap"]');
  if (script.length == 0) {
    return deferred.reject();
  }

  var src = script.attr('src');
  $.get(src).done(function(response) {
    var matches = response.match(/(?!v)([.\d]+[.\d])/);
    if (matches && matches.length > 0) {
      version = matches[0];
      deferred.resolve(version);
    }
  });

  return deferred;
};

getBootstrapVersion().done(function(version) {
  console.log(version); // '3.3.4'
});

The version of each of Bootstrap's jQuery plugins can be accessed via the VERSION property of the plugin's constructor. For example, for the tooltip plugin:

$.fn.tooltip.Constructor.VERSION // => "3.3.7"

src //getbootstrap.com/javascript/#js-version-nums

if you mean from the css, then yu ahve to AJAX the file and .match(/v[.\d]+[.\d]/);